The use of mainstream CMOS (complimentary metal-oxide semiconductor) technology for millimeter wave applications is gaining attention primarily due to its low cost. However, at millimeter wave frequencies, conventional schemes for generation of a local oscillator (LO) signal are not practical due to the very high frequency operation. Achieving good tuning range, phase noise and precise quadrature signals at millimeter wave frequencies, (e.g. 60 GHz) is not easy. Besides, the available power from a CMOS device is very low at such frequencies.
Hence, it is advantageous to have the LO source operating at a lower frequency (preferably a sub-harmonic of the required LO frequency to down convert the RF signal) because of ease of implementation, availability of better well characterized passives and better phase noise and tuning range with less susceptibility to parasitics. Such a LO source could be followed by a frequency multiplier to drive the mixer in the receive and transmit chain. Frequency multiplication also translates a small tuning range at a lower frequency to a larger tuning range at a higher frequency on an absolute scale although the relative tuning rage with respect to that of the center of the band is constant.
There are different ways of implementing a frequency multiplier. For example, a frequency multiplier may be implemented using a single FET device and terminating its terminals at unwanted harmonic components. Other frequency multipliers have been implemented either using diodes (varactors, step recovery diodes and Schottky-barrier diodes) or active devices. While passive resistive diode multipliers are broadband and inefficient, and varactors are narrowband and efficient, active multipliers can have broad bandwidths and conversion gain. The DC power advantage of active multipliers is essential for RF and wireless applications. In an active multiplier, the key to efficient operation is the prevent loss of power at any other harmonic other than the harmonic of interest.